The present invention relates to the filtration and degassing of molten metal. Molten metal, particularly molten aluminum in practice, generally contains entrained and dissolved impurities both gaseous and solid which are deleterious to the final cast product. These impurities may effect the final cast product after the molten metal is solidified whereby processing may be hampered or the final product may be less ductile or have poor finishing and anodizing characteristics. The impurities may originate from several sources. For example, the impurities may include metallic impurities such as alkaline and alkaline earth metals and dissolved hydrogen gas and occluded surface oxide films which have become broken up and are entrained in the molten metal. In addition, inclusions may originate as insoluble impurities such as carbides, borides and others or eroded furnace and trough refractories.
It is naturally highly desirable to improve the filtration and degassing of molten metals in order to remove or minimize such impurities in the final cast product, particularly with respect to molten aluminum and especially, for example, when the resultant metal is to be used in a decorative product such as a decorative trim or products bearing critical specifications such as aircraft forgings and extrusions and light gauge foil stock. Impurities as aforesaid cause loss of properties in the final cast product such as tensile strength and corrosion resistance.
Rigorous metal treatment processes such as gas fluxing or melt filtration have minimized the occurrence of such defects. However, they have not been successful in reducing the occurrence of such defects to a satisfactory level for critical applications. Conventionally conductive gas fluxing processes such as general hearth fluxing have involved the introduction of the fluxing gas to a holding furnace containing a quantity of molten metal. This procedure required that the furnace be shut down while the fluxing gas is circulated so that the metal being treated would remain constant and treatment could take place. This procedure has many drawbacks, among them, the reduced efficiency resulting from the prolonged idleness of the furnace during fluxing as well as the lack of efficiency due to the low surface area to volume ratio between the gas flux and the molten metal. Further factors comprise the restriction of location to the furnace which permitted the re-entry of impurities to the melt before casting, and the high emissions resulting from both the sheer quantity of flux required and the location of its circulation.
As an alternative to the batch-type fluxing operations employed as aforesaid, certain fluxing operations were employed in an inline manner; that is, the operation and associated apparatus were located outside the melting or holding furnace and often between the melting furnace and either the holding furnace or the holding furnace and the casting station. This helped to alleviate the inefficiency caused by furnace shut down but was not as successful in improving the efficiency of the operation itself, in that undesirably large quantities of fluxing gas were often required per unit of molten metal, which was both costly and detrimental to air purity.
Conventionally, the melt filtration is utilized in order to decrease the extent of the aforesaid defects. The most common form of melt filtration involves the use of open-weave glass cloth screens placed in transfer and pouring troughs or in the molten pool of metal in the top of a solidifying ingot. Such filters have been found to be only partially effective since they remove only the larger inclusions. Another type of filter in common use is a bed filter made up, for example, of tubular alumina. Such filters have many disadvantages, perhaps the most serious of which is the great difficulty experienced in controlling and maintaining the pore size necessary for efficient filtration. Another difficulty with such filters is their tendency to produce an initial quantity of metal having poor quality at the start up of each successive casting run.
Porous ceramic foam materials are known in the art, for example, having been described in U.S. Pat. Nos. 3,090,094 and 3,097,930. These porous ceramic foam filters are known to be particularly useful in filtering molten metal, as described in U.S. Pat. No. 3,893,917 for "Molten Metal Filter" by Michael J. Pryor and Thomas J. Gray, patented July 8, 1975, and also as described in U.S. Pat. No. 3,962,081 for "Ceramic Foam Filter" by the inventors of the present invention.
Porous ceramic foam materials are particularly useful for filtering molten metal for a variety of reasons including among which are their excellent filtration, low cost, ease of use and ability to use same on a disposal, throwaway basis. The fact that these ceramic foam filters are convenient and inexpensive to prepare and may be used on a throwaway basis requires the development of means for easily and conveniently assembling and removing the porous molten filters from a filtration unit.
Accordingly, it is a principal object of the present invention to provide an improved filter apparatus for the filtration of molten metal with the removable filter plate.
It is a particular object of the present invention to provide improved removable filter plates for use in the filtration of molten metal.
It is still a further principal object of the present invention to provide an improved method and apparatus for the filtration and degassing of molten metal which employs contact between molten metal and fluxing gas within the filter plate.
Further objects and advantages of the present invention will appear hereinbelow.